Like other urban areas, San Francisco suffers from light pollution. Nighttime lights from the city's baseball park attracts ashy storm-petrels from nests 30 miles away, on the Farallon Islands.

Photograph by Jim Richardson, National Geographic Creative

Streetlights and the light from shopping centers, stadiums, and houses turn night into day, a "loss of night" that is shifting the internal clocks of birds worldwide. Now, scientists are trying to understand how artificial lights are affecting birds' songs, mating, and reproduction.

High on bluffs overlooking the Pacific, Dominik Mosur was strolling along at 2 a.m. searching for owls. Darkness enveloped the Presidio, a historic military encampment turned national park, as Mosur made his way through cypress-scented fog.

Alert in the mist as he cut through a forest, Mosur listened for the hoot of the great horned owl. Instead, he heard the singing of a bird that should have been asleep in its nest until dawn. The Nuttall's white-crowned sparrow was throbbing away with its distinctive zu-zee trill.

To this day, Mosur wonders whether the bright streetlamps, 50 feet from the songbird's territory, caused its odd nocturnal behavior, which usually is limited to moonlit nights along this part of the coast.

Mosur puzzled over the toll that the nighttime singing was taking: Would the songbird have the energy in the morning to defend its territory, attract a mate, and raise its young?

Around the world, scientists seeking to answer that question have gathered mounting evidence that city lights are altering the basic physiology of urban birds, suppressing their estrogen and testosterone and changing their singing, mating, and feeding behaviors. One experiment showed that male blackbirds did not develop reproductive organs when they were exposed for two years to light at night.

"Birds are particularly sensitive to light and different chemical interventions. If you see these deleterious effects in the birds, you're likely to see them in humans in short order. The smart thing to do is to pay attention to avian life," said Vincent Cassone, whose University of Kentucky laboratory studies the hormones and nervous systems of birds and mammals.

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Researchers at the University of Memphis have documented hormonal changes in western scrub-jays that could interfere with reproduction.

Photograph by Marc Moritsch, National Geographic Creative

Changing Clocks

People can suffer an array of health problems when they work night shifts that alter their circadian, or daily, biological cycles. Other animals can suffer as well: In the wild, light pollution causes hatchling sea turtles to lose their way from beach to ocean, and disorients monarch butterflies searching for migration routes. In field experiments, Atlantic salmon swim too early to the sea, and frogs stop mating when the sky glows from nearby stadium lights at football games. Millions of birds die from collisions with brightly lit communication towers, and migratory flocks are confused by signals gone awry.

More recently, researchers have documented an earlier dawn chorus, which influences mate selection, feeding, and interactions between species. At a deeper, molecular level, the changes in birds' hormones raise questions about their reproductive fitness and the potential for ecological and evolutionary consequences.

The year-long cycle of light is the most important environmental cue for birds because it synchronizes their seasonal changes in physiology and behavior. Since artificial light upsets hormone levels and signals, it could disrupt their intricate clocks, which evolved to help them cope with complex environments.

"Birds can tell time in their brains. They know what time of day it is, what tomorrow is going to be like. They know where they are in the world using solar information. They can track the sun. They hear the sound of ocean breakers and wind over mountains," Shoech said.

What's more, birds can see light in the ultraviolet range, Shoech added. "They are phenomenally attuned to time and space, to the point of approaching science fiction. You don't need science fiction."

Birds have light receptors in their retinas and in their pineal glands, as well as in other parts of their brains. The pineal gland secretes the hormone melatonin at night, which guides biological clocks controlling body function, growth, and behavior. In birds, melatonin appears essential to encode and store information about time.

Daylight Hours

Like humans, birds are diurnal, mostly evolved to a cycle of daytime activity and nighttime sleep. Birds synchronize their internal clocks with light to time their daily and seasonal foraging, communication, reproduction, and migration.

Birds also depend on sounds they hear, and adjust their behavior to the rhythm of night and day. The timing of the first calls at sunrise—the dawn chorus or daybreak song—is based on changing light intensities. The end of foraging is based on darkening twilight.

But researchers caution that premature dawn calls may disrupt age-old signals for choosing mates, and expanded feeding may draw wading birds to degraded areas or to danger from predators.

"We don't know if being active at night comes with energetic costs," said biologist Davide Dominoni, who conducted research at the Max Planck Institute for Ornithology in Munich before moving on to the University of Glasgow.

Little is known about how the increase in length of day for the birds affects their fitness, reproductive success, or survival.

"There might be a physiological or biomedical cost," Dominoni said. "With humans, we are starting to realize that disrupting body clocks can really come with serious health consequences linked to immune function, metabolism, cancer, obesity, and diabetes. These types of things are relatively unexplored in wild animals."

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Far from city lights, a blackbird perches on a tree stump in England. In one experiment, male blackbirds exposed to night lights for two years did not develop reproductive organs.

Photograph by David Tipling, imageBROKER/Corbis

Altered Hormones

Seventy-five miles from the Presidio, in Davis, California, researchers tiptoed in the low bushes trapping and netting western scrub-jays. Schoech has been studying a related species, Florida scrub-jays, for more than 20 years. At this point he was investigating why the jays living in Florida suburbs were laying eggs two to four weeks earlier than jays in native wildlands.

Schoech and a post-doctoral researcher on his team, Eli Bridge, recruited energetic University of California students to help capture two dozen wild scrub-jays in Davis. After packing them carefully in modified pet carriers, they trucked the jays back to the Tennessee lab to serve as surrogates for the Florida scrub-jays, which are a protected species.

Their work ultimately found that the earlier scrub-jay eggs in Florida's suburbs probably had nothing to do with artificial light; Schoech thinks it's due to the abundance of food in suburbs. But in the course of the work, the study led to new lab experiment findings about how night light affects hormones.

Schoech's experiment was probably the first to study birds' sex hormones in both females and males in response to realistic exposures mimicking suburban lights. He demonstrated that artificial light tended to dampen the secretion of reproductive hormones, confirming limited findings by others.

The team found that reductions of sex hormones—estradiol and testosterone—occurred at different times in females and males, creating a mismatch between the sexes in the cues that synchronize biological rhythms.

"Light at night disrupted the extraordinarily strong correlation between testosterone and estradiol in jays of both sexes that existed under unlighted nocturnal conditions," Schoech said.

In the natural darkness of night, birds' reproductive hormones rise as winter turns to spring and days lengthen. This did not occur among the jays exposed to night light. Instead, "light at night interfered with endocrine responses to increased day length," Schoech said.

In Beijing, scientists published findings this year similar to Schoech's, concluding that urban tree sparrows exposed to artificial night light began to secrete a reproductive hormone earlier than rural tree sparrows did. The urban birds' estradiol and testosterone levels were lower than those of rural birds.

Male blackbirds exposed in a laboratory to light at night developed testes faster in the first year. "During the second year, the reproductive system did not develop at all," according to the report by Dominoni and colleagues published last year.

"The rub is how these endocrine changes translate into reproductive fitness. We frankly don't know. It's too early. It's a reasonable thing to say that it's disturbing a natural process," said Cassone of the University of Kentucky.

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Streetlights in the town of Harmony, Florida, are shaded so that light levels remain low at night. The town doesn't even allow lighted tennis courts that would mar the night sky.

Photography by Jim Richardson

Night Skies Aglow

Astronomers were the first to caution about the harm that comes from sky glow, since it blocked views of faint celestial objects. Then came NASA photos of Earth from space: first the image of the Blue Marble, showing a small finite planet, then decades later, the Black Marble, showing the night sky invaded with artificial lights.

Two-thirds of the world, and 99 percent of the United States' lower 48 states and the European Union, live under conditions of light pollution. City lights can be 4.8 times brighter than the natural night sky, according to European scientists. In Vienna and Plymouth, England, the natural cycles of moon brightness are close to extinct.

In 2001, scientists reported that one-fifth of the world population, and more than two-thirds of the U.S. population, can't see the Milky Way with the naked eye.

"You can see how artificial light is spreading over the world, as much as a 20 percent increase a year in some geographic regions," said Reinhard Klenke, a biologist at the Helmholtz Centre for Environmental Research in Leipzig, Germany.

On San Francisco's clear Indian summer nights of September and early October, Mosur, a local bird expert, sets his spotting scope at the moon, hoping to catch silhouettes of warblers, thrushes, and sparrows.

"That's when birders hope to hear, if not glimpse, the nocturnal migration of songbirds passing over on the way south," Mosur said.

Like any big city, San Francisco struggles with lights. The bright lights of the Giants' baseball park attract ashy storm-petrels from nests 30 miles away, on the Farallon Islands. Residents argue over whether putting up lights on a soccer field in Golden Gate Park will interfere with songbird migrations.

Already the city sky has changed.

"I've noticed the night sky looks a lot different when I'm standing on Twin Peaks in the city or on a ridge near Lake Hennessey 20 miles north of Napa. There's a big difference in what you see in the sky," he said. "I'm sure the birds see the differences, too."